Method of Optical Fabrication of Three-Dimensional Polymeric Structures With Out of Plane Profile Control

ABSTRACT

A method of optical fabrication comprises coating a substrate with a photocuring material, controlling the application of light to the photocuring material so as to control the intensity and pattern of the light both in-plane and out of plane, and developing the photocuring material.

This application is a continuation of U.S. application Ser. No.12/083,135, filed 14 Oct. 2008 and entitled Method Of OpticalFabrication Of Three-Dimensional Polymeric Structures With Out Of PlaneProfile Control, the entirety of which is hereby incorporated byreference. This application claims the benefit of U.S. application Ser.No. 60/723,082, filed 03 Oct. 2005 and entitled Optical Fabrication OfThree-Dimensional Polymeric Microstructures, the entirety of which ishereby incorporated by reference. This application also claims thebenefit of PCT application serial no. PCT/US2006/038468, filed 03 Oct.2006 and entitled Method Of Optical Fabrication Of Three-DimensionalPolymeric Structures With Out Of Plane Profile Control, the entirety ofwhich is hereby incorporated by reference.

BACKGROUND

The present invention is related to maskless fabrication techniques and,more particularly, to techniques that rely on optical curing of apolymeric material.

Physics based techniques have created a revolution in the computer chipindustry and have been further developed in sophisticated applicationsusing silicon and glass for building traditional integrated circuits(IC) through processes including photolithography, etching techniques,and deposition. These processes often have limitations includingfabrication cost, clean-room conditions, labor intensive processes, andmaterial technologies. Thus, alternate technologies have evolvedincluding soft lithography, nanoimprint lithography, microcontactprinting, and capillary lithography. One of the common features of manyof these techniques is that they utilize a mold of either silicon orpolymer, which is brought into contact with an underlying maskcontaining the essential fabricated features. These systems requireaccess to specific equipment and also significant fabrication time foreach component. The original process of hard lithography has beenaugmented with the advent of soft lithography, which allowed a mastermold to be developed and then used to produce many copies withpolydimethylsiloxane or other polymers.

One example of a maskless optical fabrication system is disclosed inU.S. Pat. No. 6,841,340 to Tani. Tani discloses a highly accuratestructure optically fabricated simply and in a short time. Rough opticalfabrication using an ultraviolet-irradiation optical fabrication processis carried out for a photo-curing resin by emission of a laser beam froma first light source, and thereafter, fine optical fabrication using atwo-photon absorption optical fabrication process is carried out byemission of a laser beam from a second light source. As a result, it ispossible to realize optical fabrication which allows fabrication of afine structure using a two-photon absorption optical fabrication processwhile realizing high speed processing using the ultraviolet-irradiationoptical fabrication process.

Another example of a maskless optical fabrication system is disclosed inU.S. Pat. No. 6,410,213 to Raguin et al. Disclosed therein is a systemfor the fabrication of arbitrary profile micro-optical structures(lenses, gratings, etc.) and, if desired, with optomechanical alignmentmarks simultaneously during fabrication, upon the use of low-contrastphotosensitive material that, when exposed to a spatially variableenergy dosage of electromagnetic radiation, can be processed to achievemulti-level or continuous surface-relief microstructures. By varying theexposure dose spatially based upon predetermined contrast curves of thephotosensitive material, arbitrary one-dimensional (1-D) ortwo-dimensional (2-D) surface contours, including spherical, aspherical,toroidal, hyperbolic, parabolic, and ellipsoidal, can be achieved withsurface sags greater than 15 μm. Surface profiles with advanced phasecorrection terms (e.g., Zernike polynomials) can be added to increasethe alignment tolerance and overall system performance of the fabricatedstructure can also be fabricated. The continuous-relief pattern can beused as is in the photosensitive material, transferred into theunderlying substrate through an etch process, electroformed into ametal, or replicated into a polymer.

SUMMARY

A technique is disclosed for building three-dimensional structures usingoptical methods combined with photocuring chemistry. This method mergesan optics based approach with chemical restructuring through thetransition of materials from distinct phases. By activating thisphotocurable material in combination with controlling the intensitydistributions that are inherently in optical patterns, in-situfabrication of three-dimensional polymeric microstructures is achieved.This simple approach combined with thermal control can create complexshapes including curved and asymmetric profiles.

According to one embodiment, a method of optical fabrication comprisescoating a substrate with a photocuring material; controlling theapplication of light to the photocuring material so as to control theintensity and pattern of the light both in-plane (e.g. X and Y) and outof plane (e.g. Z); and developing the photocuring material. Otherembodiments include controlling the application of the light with one ormore lenses, by combing one or more light beams, or some combination oflenses and light beams. Developing the photocuring material may involvecontrolling the time and or temperature. According to anotherembodiment, the structure may be heated to use thermal reflow to helpshape the device.

This method has potential applications in a variety of fields includingoptical techniques, photoactivatable materials, and lab-on-a-chipsystems. Those, and other advantages and benefits will be apparent fromthe description below.

BRIEF DESCRIPTION OF THE FIGURES

For the present invention to be easily understood and readily practiced,the present invention will now be described, for purposes ofillustration and not limitation, in conjunction with the followingfigures wherein:

FIG. 1 is a block diagram of a system according to the teachings of thepresent invention.

FIGS. 2A and 2B illustrate a maskless fabrication process.

FIG. 3A illustrates wall profiles of the structures based on theoreticalmodeling of the SU-8 material comparing the normalized width to theheight of the systems. FIG. 3B is a scanning electron microscope (SEM)image of fabricated structures. FIG. 3C illustrates a plurality of SU-8microstructures fabricated using the curing process with a mask for apoint of comparison (light source: inverted epifluorescence microscope).FIG. 3D illustrates the relationship between curing time and the heightof SU-8 microstructures.

FIG. 4 is a profile control of SU-8 microstructures built using themaskless curing process (light source: laser from a confocalmicroscope). Faster curing times control the uniformity of athree-dimensional scanned system for SEM images of a FIG. 4A small andFIG. 4B large vertical actuation of the laser source in the confocalmicroscope. FIG. 4C is an SEM image of a three-dimensional structurefabricated through controlling the horizontal and vertical scanningfocus position. The curved surface is observed by the profilometermeasurements in FIG. 4D.

FIG. 5 illustrates the control of thermal reflow with the masklessprocess to form donut shaped rings. SEM images and profilometermeasurements for curing times of FIGS. 5A and 5D—1 sec; FIGS. 5B and5E—10 secs; and FIGS. 5C and 5F—60 secs.

FIG. 6A illustrates a beam of light, while FIGS. 6B and 6C illustratethe in-plane and out of plane profiles, respectively, of the beam oflight in FIG. 6A.

FIG. 7 is a block diagram illustrating the process flow of the disclosedmethod.

DESCRIPTION OF A PREFERRED EMBODIMENT

In this document, we introduce a novel process for creatingthree-dimensional structures that can be practiced on a variety ofdevices, depicted generally in FIG. 1. In FIG. 1, the system 10 iscomprised of a light source 12, a lens 14, and a movable stage 16. Thestage 16 and/or the lens 14 can be moved via X, Y, and Z actuators 18.The position of the stage 16 and/or the lens 14, the intensity andoptionally the wavelength of the light source 12, and the focus of thelens 14 are controlled by a controller 20 executing softwareinstructions for carrying out the process.

One example of the system 10 depicted in FIG. 1 utilizes an invertedmicroscope (not shown) with either epi-fluorescence or laser confocalcapabilities. The source of excitation is used to activate thephotocurable material, inducing a shift to a stable solid state. Usingthis technique and dictating the excitation profile through controllingthe X, Y, and Z distributions with the actuation of the microscope stageand objective, we build three-dimensional microstructures with variousconfigurations on the same substrate. Patterns, intensities and the likein the X, Y directions may be referred to as in-plane while patterns,intensities and the like in the Z direction may be referred to as out ofplan.

This maskless fabrication process creates easily controlled profiles byimplementing the optical principles of spatial signal distribution.Intensities along the X, Y, and Z axes are not step functions, butrather have a distribution across all of the spatial dimensions evenwhile focusing with optical objectives. Our technique also utilizes thefeature of thermal distributions in the horizontal and verticalconfigurations as this allows additional control over creatingasymmetric profiles during the photocuring process.

SU-8 is used as the photocurable material for fabricating polymericmicrostructures using these optical techniques. SU-8 is a negative,epoxy-type, near-UV photoresist based on EPON SU8 resin (ShellChemical). We prepare the material by dissolving an EPON resin SU8 in anorganic solvent gamma-butyrolacton (GBL) with the amount of the solventdetermining the viscosity and the range of potential thicknesses of thefilm. The material in our studies consists of GBL (59% wt.) and solids(41% wt.). After spinning the liquid SU-8 resin on the substrate, weheat the system on a hotplate at 75° C. for 30 minutes (softbake). Wethen utilize a microscope to expose the SU-8 to UV excitation witheither a laser confocal microscope (Olympus Fluoview 1000) or anepi-fluorescence microscope (Zeiss Axiovert) with the light sourcespectrum between 365 nm and 410 nm. We then develop the system inpropylene glycol methyl ether acetone (PGMEA) and rinse in waterfollowed by air drying. We use a maskless process to fabricate thesethree-dimensional systems, which involve locally activating thephotocurable material.

FIGS. 2A and 2B are schematics of the disclosed maskless fabricationprocess. A glass substrate 22 is coated with photocurable SU-8 resin 24and is excited in localized areas by either a scanning laser confocalmicroscope, an epifluorescence inverted microscope, or other appropriatelight source. The spectrum of the light source is between 365 nm and 410nm. The substrates are then developed with propylene glycol methyl etheracetone to create the final structures.

To control the creation of the two- and three-dimensional structures inour process, the distribution of the excitation must be understood. Wepredict the wall profile of the polymeric microstructure based on theintensity of the signals to which the material would be exposed. Theattenuation of intensity in the SU-8 in the vertical direction (which iscontrolled by the focal plane of the microscope objective) can beapproximated by a power function with respect to depth. The attenuationfunction of near UV-light intensity in SU-8, B(r), is represented by:

B(r)=1−C×r ^(D)  (1)

where r is the depth in the SU-8 in micrometers. The constants C and Din Eq. (1) are 0.066 and 0.403, respectively.

Furthermore, a contour of the normalized constant intensity in the SU-8can be obtained through:

$\begin{matrix}{\frac{{I_{F}\left( {x,r} \right)}{B(r)}}{k\; I_{inc}} = {B\left( {r = D} \right)}} & (2)\end{matrix}$

where k is the inverse product of exposure dosage and exposuresensitivity, I_(F)(x) is the intensity distribution pattern and I_(inc)is the intensity of incident UV-light. With a given material, selectingthe proper exposure time yields a corresponding parameter k. FIG. 3A isthe SU-8 wall profile predicted by Eq. (2). Based on the theoreticalsolution, the width at the top of the SU-8 would be smaller than at thebase (closer to the surface), which is confirmed by a scanning electronmicroscope (SEM) image in FIG. 3B. We use conventional mask lithography(for comparison) with UV light activation, which reveals repeatable, yetundulating patterns (see FIG. 3C). This pattern suggests that a thermaleffect generates a reflow of the photocurable material, which could beused in the creation of applications such as a functional biomimeticmicrolens arrays. In addition, controlling the curing times of thesesystems can be leveraged to define the heights of the individualfeatures. (see FIG. 3D) The thermal reflow effect in this process thoughcan be reduced through using a laser with low power but high energydensity to cure the SU-8 material. The thermal reflow may beimplemented, before, after, or during the developing step, or somecombination thereof.

FIGS. 4A and 4B exhibit the polymeric microstructures that are builtusing the laser of a confocal microscope. Depending on the exposuretime, these structures have constant edges yet are different in heightthrough vertical scanning of the laser. A three-dimensionalmicrostructure with a curved surface similar to the surface of anoptical lens is fabricated when controlling the focus of the laser (FIG.4C) in all three axes; this is quantitatively determined using aprofilometer (FIG. 4D). Increasing the curing times allows us to obtaintaller structures in the vertical direction with the tightest toleranceon the curing time occurring below approximately 40 seconds.

The thermal effect in the maskless curing process allows another degreeof control in three-dimensions. The curing time affects the outer ringwhen curing this surface based SU-8. With an increase in the curingtime, the outer ring is enlarged. At less than 10 seconds, thepolymerization of SU-8 occurs with a relatively uniform distribution(FIGS. 5A and 5D). At longer times for heating, the thermal effectinduced from the UV source allows the temperature in the center of thisprofile to become higher than at the outer edge, which results in thereflow and spreading of the SU-8 (FIGS. 5B and 5E). At greater than 60seconds, the outer ring becomes the dominant feature across the width(FIGS. 5C and 5F).

Turning now to FIG. 6A, FIG. 6A illustrates a beam of light 30 that maybe used to activate the photocurable material. The intensity and thewavelength of the beam 30 may be controlled by sending appropriatecontrol signals from the controller 20 to the light source 12 of FIG. 1.The in-plane profile (shape) of the beam of light is shown in FIG. 6Bwhile the out of plane profile (shape) of the beam of light is shown inFIG. 6C. By controlling the out of plane profile, the profile (shape) ofthe walls of the structure being fabricated can be controlled. The outof plane profile of the beam of light can be controlled by controllingthe lens 14 of FIG. 1. Another way to control the out of plane profileis to use more than one light beam, and to have those beams interferepositively (so as to add to one another) or negatively (so as tosubtract from one another). Some combination of multiple beams andmultiple lenses may also be used.

FIG. 7 is a block diagram illustrating the steps performed in carryingout an embodiment of the process disclosed herein. In step 40, asubstrate is coated with a photocuring material. (See FIG. 2A). Thephotocuring material is typically applied to the substrate in liquidform and spun to provide a uniform coating. In step 42, the coating ismade less viscous, typically by baking (heating). However, depending onthe photocuring material being used, this step may be eliminated.Typically, the final shift to a solid state is accomplished by theapplication of light at step 44.

The next step 44 is to control the application of light to thephotocuring material. (See FIG. 2A) The control can take many forms asdiscussed above in conjunction with FIG. 6, e.g. controlling one or moreof the intensity, the frequency, the in-plan profile and the out ofplane profile depending on the structure to be fabricated. Step 44 maybe referred to as activating the photocuring material.

Step 46 represents an optional thermal reflow step. The structurescreated by selective activation of the photocuring material may beheated in step 46 through any appropriate means to achieve a thermalreflow of the material. At step 48 the photocuring material isdeveloped. In some systems, the photocuring material that has beenexposed to the light is not removed in the developing step whilephotocuring material that has not been exposed to the light is removed.In other systems, the opposite occurs. In any event, by controlling thedeveloping time as well as the developing temperature, the ability toinfluence the shape of the structure through thermal reflow may beachieved at the same time that developing occurs.

Another optional thermal reflow step 50 may be applied. Like the otherthermal reflow steps, the application of heat in any suitable manner maybe used to shape of the structure by causing certain of the material toreflow as a result of the applied heat.

Developing simplified systems for the creation of more complexstructures will create greater utility for researchers in many fields.This simplicity will also allow additional technologies to beimplemented in combination with our method. We have demonstrated athree-dimensional process using laser confocal and invertedepifluorescence microscopes, which relies on simple optical methodscombined with photoactivatable materials. We can fabricatemicrostructures with various profiles and heights through control of theheat distribution or the focus for light activation. Through thistechnology, a wide range of applications will benefit including opticaltechniques and technologies such as microfluidics and lab-on-a-chip aswell as biologically inspired structures.

It should be recognized that the above-described embodiments of theinvention are intended to be illustrative only. Numerous alternativeembodiments may be devised by those skilled in the art without departingfrom the scope of the following claims.

1. A method of optical fabrication, comprising: coating a substrate witha photocuring material; controlling the application of light to thephotocuring material so as to simultaneously control the intensity andpattern of the light both in-plane and out of plane; and developing thephotocuring material.
 2. The method of claim 1 wherein said photocuringmaterial is an epoxy-type, near-UV photoresist.
 3. The method of claim 1wherein said controlling the application of light in-plane comprisescontrolling the application of light in an X direction and in a Ydirection.
 4. The method of claim 1 wherein said controlling theapplication of light out of plane comprises controlling the applicationof light in a Z direction.
 5. The method of claim 1 additionallycomprising controlling the wavelength of the light.
 6. A method ofoptical fabrication, comprising: coating a substrate with a photocuringmaterial; heating the photocuring material; controlling the applicationof light to the photocuring material so as to simultaneously control theintensity and pattern of the light in each of an X, a Y, and a Zdirection to form structures; heating certain structures to causethermal retlow; and developing the photocuring material.
 7. The methodof claim 6 wherein said heating certain structures is performed at leastone of before or during said developing.
 8. The method of claim 6wherein said photocuring material is an epoxy-type, near-UV photoresist.9. The method of claim 6 wherein said controlling the application oflight in the X and Y directions controls a top profile of fabricatedstructures.
 10. The method of claim 6 wherein said controlling theapplication of light in the Z direction controls a profile of the wallsof the fabricated structures.
 11. The method of claim 6 additionallycomprising controlling the wavelength of the light.
 12. A method ofoptical fabrication, comprising: coating a substrate with a photocuringmaterial; reducing the viscosity of the photocuring material; activatinga desired pattern within said photocuring material by simultaneouslycontrolling the application of light in each of an X, a Y, and a Zdirection; and developing the photocuring material.
 13. The method ofclaim 12 wherein said controlling the application of light includescontrolling a stage and an objective of an epiflourescence microscope ora laser confocal microscope.
 14. The method of claim 12 wherein saidreducing the viscosity includes heating said photocuring material. 15.The method of claim 12 additionally comprising heating a portion of saiddesired pattern to cause thermal reflow of said photocuring material.16. The method of claim 15 additionally wherein said heating to causethermal reflow is performed at least one of before or during saiddeveloping.
 17. The method of claim 1 wherein said controlling theapplication of light includes controlling a stage and an objective ofthe epiflourescence microscope or the laser confocal microscope.
 18. Themethod of claim 6 wherein said controlling the application of lightincludes controlling a stage and an objective of the epiflourescencemicroscope or the laser confocal microscope.
 19. The method of claim 6wherein said heating certain structures is performed after saiddeveloping.
 20. The method of claim 16 additionally wherein said heatingto cause thermal reflow is performed after said developing.